1. Field of the Invention
[1] This invention relates to an iron-based powder which is suitable for use in various high strength sintered components. Specifically, this invention relates to an alloyed steel powder that can undergo re-compaction under a light load when it is applied to re-compaction of sintered powder preforms.
2. Description of the Related Art
[2] Powder metallurgical technology can produce a component having a complicated shape as a xe2x80x9cnear net shapexe2x80x9d with high dimensional accuracy and can markedly reduce the cost of cutting and/or finishing. In such a near net shape, almost no mechanical processing is required to obtain or form a target shape. Powder metallurgical products are, therefore, used in a variety of applications in automobiles and other various fields. For miniaturization and reduction in weight of components, demands have recently been made on such powder metallurgical products to have higher strength. Specifically, strong demands have been made on iron-based powder products (sintered iron-based components) to have higher strength.
[3] A basic process for producing a sintered iron-based component (sometimes hereinafter referred to as xe2x80x9csintered iron-based compactxe2x80x9d or simply as xe2x80x9csintered compactxe2x80x9d) includes the following sequential three steps (1) to (3): (1) a step of adding a powder for an alloy such as a graphite powder or copper powder and a lubricant such as zinc stearate or lithium stearate to an iron-based powder such as an iron powder or alloy steel powder to yield an iron-based mixed powder; (2) a step of charging the iron-based mixed powder into a die and pressing the mixed powder to yield a green compact; and (3) a step of sintering the green compact to yield a sintered compact. The resulting sintered compact is subjected to sizing or cutting according to necessity to thereby yield a product such as a machine component. When the sintered compact requires higher strength, it is subjected to heat treatment such as carburization or bright quenching and tempering. The resulting green compact obtained through the steps (1) to (2) has a density of at greatest from about 6.6 to about 7.1 Mg/m3.
[4] In order to further increase the strength of such iron-based sintered components, it is effective to increase the density of the green compact to thereby increase the density of the resulting sintered component (sintered compact) obtained by subsequent sintering. The component with a higher density has fewer pores and better mechanical properties such as tensile strength, impact value and fatigue strength.
[5] A warm compaction technique, in which a metal powder is pressed while heating, is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404 and U.S. Pat. No. 5,368,630 as a process for increasing the green density. For example, 0.5% by mass of a graphite powder and 0.6% by mass of a lubricant are added to a partially alloyed iron powder in which 4 mass % Ni, 0.5 mass % Mo and 1.5 mass % Cu are contained, to yield an iron-based mixed powder. The iron-based mixed powder is subjected to the warm compaction technique at a temperature of 150xc2x0 C. at a pressure of 686 MPa to thereby yield a green compact having a density of about 7.3 Mg/m3. However, the density of the resulting green compact is about 93% of the density, and a further higher density is required. Additionally, application of the warm compaction technique requires facilities for heating the powder to a predetermined temperature. This increases production cost and decreases dimensional accuracy of the component due to thermal deformation of the die.
[6] The sinter forging process, in which a green compact is directly subjected to hot forging, is known as a process for further increasing the density of a green compact. The sinter forging process can produce a product having a substantially true density but raises the cost beyond the other powder metallurgical processes, and the resulting component exhibits decreased dimensional accuracy due to thermal deformation.
[7] As a possible solution to these problems, Japanese Unexamined Patent Application Publications No. 1-123005 and No. 11-117002 and U.S. Pat. No. 4,393,563, for example, propose a technique that can produce a product having a substantially true density as a combination of the powder metallurgical technology and re-compaction technology such as cold forging (the proposed technique is sometimes hereinafter referred to as xe2x80x9cre-compaction of sintered powder preformsxe2x80x9d). FIG. 3 shows an example of an embodiment of a production process of a sintered iron-based component using the re-compaction of sintered powder preforms.
[8] With reference to FIG. 3, raw material powders such as a graphite powder and a lubricant are mixed with an iron-based material powder to yield an iron-based powder mixture. Next, the iron-based powder mixture is subjected to compaction to yield a preform, followed by preliminary sintering of the preform to yield a sintered iron-based powder metal body. Next, the sintered iron-based powder metal body is subjected to re-compaction such as by cold forging to yield a re-compacted body. The resulting re-compacted body is then subjected to re-sintering and/or heat treatment to thereby yield a sintered iron-based component.
[9] This technique using re-compaction of sintered powder preforms is intended to increase the mechanical strength of the product (sintered iron-based component) by subjecting the sintered iron-based powder metal body to re-compaction to thereby increase the resulting density to a value near the true density. This technique can produce a component having high dimensional accuracy since there is less thermal deformation in the re-compaction step. However, to produce a sintered product having high strength by using this technique, (1) the sintered iron-based powder metal body must have high deformability and must be able to undergo re-compaction under a light load, and concurrently, (2) the sintered iron-based component after re-sintering and/or heat treatment must have high strength.
[10] Separately, elements for improving quenching property are generally added to a iron-based powder to improve the strength of a sintered iron-based component.
[11] For example, Japanese Examined Patent Application Publication No. 7-51721 mentions that, when 0.2 to 1.5% by mass of Mo and 0.05 to 0.25% by mass of Mn are prealloyed to an iron powder, the resulting sintered compact can have a high density without substantially deteriorating compressibility during compaction.
[12] Japanese Examined Patent Application Publication No. 63-66362 discloses a powder metallurgical alloyed steel powder composed of an atomized alloyed steel powder and a powder (particle) of at least one of Cu and Ni partially diffused and bonded to a surface of the atomized alloyed steel powder, which atomized alloyed steel powder contains prealloyed Mo within a compositional range that does not adversely affect. the compressibility of the powder. The publication mentions that this alloyed steel powder comprises prealloyed Mo and partially alloyed Cu or Ni to thereby concurrently obtain high compressibility during compaction and high strength of the component after sintering.
[13] The alloyed steel powder described in Japanese Examined Patent Application Publication No. 63-66362 comprises partially alloyed Ni and/or Cu among alloying elements to ensure compressibility during compaction. However, Ni and Cu are highly diffusible into a steel powder matrix and diffuse into the steel powder matrix during preliminary sintering when the alloyed steel powder is subjected to a re-compaction of sintered powder preforms process. Accordingly, the resulting sintered iron-based powder metal body obtained through the provisional sintering step has a high hardness and requires a high load for re-compaction.
[14] Likewise, the alloyed steel powder (iron-based powder) described in Japanese Examined Patent Application Publication No. 7-51721 is a prealloyed powder, and when this is subjected to re-compaction of sintered powder performs process, the resulting sintered iron-based powder metal body obtained through preliminary compaction and preliminary sintering has a high hardness and requires a high load for re-compaction. Consequently, the costs of facilities for re-compaction are increased or the life of the die is shortened.
[15] Accordingly, the purpose of this invention is to provide an alloyed steel powder with excellent compressibility. This can solve the problems of the above mentioned conventional technologies, This can decrease the hardness of a sintered iron-based powder metal body obtained through compaction and preliminary sintering, can minimize the re-compaction load, and can increase the strength of a sintered iron-based component produced through re-sintering and/or heat treatment.
[16] After intensive investigations on the composition of an iron-based material powder (iron-based powder) that is suitable for re-compaction of sintered powder preforms process, we have found that, when an iron-based powder contains prealloyed Mn and optionally Mo, based on the entire amount of said alloyed steel powder in an amount less than or equal to a predetermined amount, and contains Mo partially diffused and bonded to a surface of the iron-based powder within a predetermined range, the use of the iron-based powder, upon re-compaction of sintered powder preforms process, markedly decreases the re-compaction load and produces a sintered iron-based component after re-compaction and/or heat treatment which has high strength.
[17] This invention has been accomplished based on these findings.
[18] Accordingly, this invention provides an alloyed steel powder, including an iron-based powder and from about 0.2 to about 10.0% by mass of Mo in the form of a powder being partially diffused and bonded to the surface of the iron-based powder particles, which iron-based powder includes about 1.0% by mass or less of prealloyed Mn with the balance substantially consisting of iron.
[19] This invention also provides an alloyed steel powder, including an iron-based powder and from about 0.2 to about 10.0% by mass of Mo in the form of a powder being partially diffused into and bonded to a surface of the iron-based powder particles, which iron-based powder includes about 1.0% by mass or less of prealloyed Mn and less than about 0.2% of prealloyed Mo with the balance substantially consisting of iron.